The internal surfaces of numerous structures are typically subjected to excessively high wear. In one example of such structures, i.e., drill bushings, the structures are used to guide drills or reamers on automated machines. Because it is critical to maintain the hole locations within specified tolerances, the structures themselves have very tight tolerances, typically 0.0002" on the inner and outer diameters and 0.0003" on concentricity between the two diameters. They are most commonly employed in the automotive and aerospace industries, but they have much wider applicability. Over 95% of the bushings sold currently are made out of 1144 carbon steel hardened to 500 to 600 kgf/mm.sup.2 by carburizing before finish machining. They undergo abrasive wear by machined metal chips that flow in the space between the drill flutes and the bushings. Thus, they must be frequently replaced which results in costly downtime. The industries have been looking for reliable alternatives that would increase the life of drill bushings while maintaining their dimensional tolerances.
Many techniques have been employed for lengthening the wear life of drill bushings. Such techniques include replacing the 1144 carbon steel bushings with cemented tungsten carbide bushings either in monolithic form or as sleeves slip-fit into steel bodies. Cemented carbide is much harder than 1144 carbon steel, with a Vickers hardness of about 2000 kgf/mm.sup.2, and provides better wear resistance. However, monolithic cemented carbide bushings are very brittle and break when misaligned drill robots hit them with the drills. Although the sleeve-type cemented carbide bushings last about four times as long as carbon steel bushings, they are about four times as expensive.
Surface hardening techniques have been used on bushings made out of either tool steel or 1144 carbon steel to improve their life. For example, case hardening have been commonly applied, but such techniques do not provide sufficient improvement in life to meet the current needs of industry. Hard coatings such as chemical vapor deposited titanium carbide and titanium nitride have also been tried with little success because of the degradation of the mechanical properties and deformation of the parts by high temperatures (.about.900.degree. C.) used during the deposition process.
The thermochemically deposited coatings described in U.S. Pat. No. 4,008,976 issued Feb. 22, 1977 to Robert A. Holzl have also been tried to prolong the life of drills and other cutting tools. Although very thin coatings described in this patent have been attempted on cutting tools and the like, such coatings have not been successful. This is true because such coatings are deposited at very high temperatures, causing degradation of the mechanical properties and deformation of the parts.
U.S. Pat. No. 4,162,345, issued July 24, 1979 to Robert A. Holzl, discloses a method for producing deposits characterized by a structure which is free of columnar grains and instead consists essentially of fine, equiaxial grains. These deposits have unusually high hardness and tensile strength. However, this patent discloses use of temperatures varying from 650.degree. C. to 1100.degree. C., which are high enough to degrade the mechanical properties and deformation (or mechanical distortion) of metallic substrates. The material of Holzl '345 patent is a hard metal alloy, consisting primarily of tungsten and carbon. X-ray diffraction analysis of the '345 alloy shows that the deposit is akin to tungsten but with a very finely dispersed carbide, probably in the form of WC.
U.S. Pat. No. 4,427,445, issued Jan. 24, 1984 to Robert A. Holzl, et al. also discloses a hard fine grained material which can be produced by thermochemical deposition, but at temperatures lower than those described in the '345 patent. Thus, where there are large differences in the thermal coefficients of expansion between the substrate material and the coating material, the '445 methodology reduces adhesion problems and problems associated with mechanical distortion, metallurgical transformation or stress relief of the substrate. The material of the '445 Holzl, et al. patent is a tungsten carbon alloy consisting primarily of a two phase mixture of substantially pure tungsten and an A15 structure.
U.S. Pat. No. 3,368,914, discloses a process for adherently depositing tungsten carbide of substantial thickness on steel and other metal substrates. The process involves first diffusing another metal on the surface of the substrate to relax the thermal expansion coefficient zone of the metal substrate. The carbide coating is then deposited on the diffused surface by chemical vapor deposition. The process claims it is preferable to diffuse the metal forming the carbide into the substrate. In one embodiment of the claimed process, a thin layer of tungsten is deposited on the metal surface using a temperature of 600.degree.-1000.degree. C. After coating tungsten, the temperature is increased to approximately 1000.degree.-1200.degree. C. and held there for a significant period of time to permit diffusion of tungsten into the metal. The diffused surface is then coated with tungsten carbide using WF.sub.6, CO and H.sub.2. In the alternative embodiment, a pack diffusion technique is used for achieving diffusion of tungsten into metal. Temperature ranging from 1000.degree.-1200.degree. C. is used in the pack diffusion step. The diffused metal surface is then coated with tungsten carbide. Since a temperature ranging from 1000.degree.-1200.degree. C. is used during the process, the '914 process is not suitable for providing erosion and abrasion wear resistance coatings on various metallic substrates without severely distorting and degrading their mechanical properties.
U.S. Pat. No. 3,389,977, discloses a method of depositing substantially pure tungsten carbide in the form of W.sub.2 C, free from any metal phase. Pure W.sub.2 C is deposited on a substrate by reacting WF.sub.6 and CO. The substrate is heated to a temperature in excess of 400.degree. C. The adherence of W.sub.2 C to steel is improved by first cleaning the surface and then depositing with a thin film of tungsten followed by W.sub.2 C using a temperature ranging from 600.degree.-1000.degree. C. Since initial deposition of tungsten is conducted at or above 600.degree. C., the '977 process is not suitable for providing erosion and abrasion wear resistance coating on metallic substrates without severely degrading their mechanical properties and deforming the structure. Additionally pure W.sub.2 C deposited according to the teachings of the '977 patent consists of columnar grains. The '977 patent does not describe a process for depositing W.sub.2 C coating in non-columnar and substantially layered fashion.
U.S. Pat. No. 3,574,672 discloses a process for depositing W.sub.2 C by heating a substrate to a temperature between 400.degree.-1300.degree. C. The process described in this patent is essentially the same as disclosed in U.S. Pat. No. 3,389,977.
U.S. Pat. No. 3,721,577 discloses a process for depositing refractory metal or metal carbides on ferrous and non-ferrous base materials heated to at least 1050.degree. C. The metal carbides are deposited using halide vapors of the metal along with methane and hydrogen. This process is not suitable for applications where structural tolerances are very tight.
U.S. Pat. No. 3,814,625 discloses a process for the formation of tungsten and molybdenum carbide by reacting a mixture of WF.sub.6 or MoF.sub.6, benzene, toluene or xylene and hydrogen. The process is carried out under atmospheric pressure and temperatures ranging from 400.degree.-1000.degree. C. An atomic ratio of W/C in the gaseous mixture varying from 1 to 2 is required to yield W.sub.2 C. The process also suggests that for some substrates such as mild steel, it is advantageous in providing better adhesion to deposit a layer of nickel or cobalt prior to tungsten carbide deposition. The process also claims the formation of a mixture of tungsten and tungsten carbide in the presence of large proportions of free hydrogen. The mixture of W and W.sub.2 C coating deposited according to the teaching of the '625 patent consists of columnar grains. The '625 patent does not disclose a process for depositing a mixture of W and W.sub.2 C in non-columnar and substantially layered fashion.
British Patent 1,326,769 discloses a method for the formation of tungsten carbide by reacting a mixture of WF.sub.6, benzene, toluene or xylene and hydrogen under atmospheric pressure and temperatures ranging from 400.degree.-1000.degree. C. The process disclosed in this patent is essentially the same as disclosed in U.S. Pat. No. 3,814,625.
British Patent No. 1,540,718 discloses a process for the formation of W.sub.3 C using a mixture WF.sub.6, benzene, toluene or xylene and hydrogen under sub-atmospheric pressure and temperatures ranging from 350.degree.-500.degree. C. An atomic ratio of W/C in the gaseous mixture varying from 3-6 is required to yield W.sub.3 C. The coating deposited according to the teaching of British Patent 1,540,718 consists of columnar grains. The British '718 patent does not teach a process for depositing a non-columnar and substantially layered coating.
Although the methods of the Holzl patents cited above have been useful in producing fine-grained tungsten-carbon alloys containing mixtures of W and WC, and W and A15 structure, and the methods described in other cited patents have been successful in producing columnar W.sub.3 C or W.sub.2 C or mixtures of W and W.sub.2 C, no one has yet disclosed a method for producing extremely hard, fine-grained, non-columnar tungsten-carbon alloys with substantially layered microstructure containing mixtures of tungsten and tungsten carbide in the form of W.sub.2 C or W.sub.3 C or a mixture of W.sub.2 C and W.sub.3 C.
In co-pending U.S. Application Ser. No. 07/092,809, now U.S. Pat. No. 4,874,642, a method and coating are described comprising a non-columnar, fine grained having a substantially layered microstructure deposit of tungsten carbide in the form of W.sub.2 C, W.sub.3 C or mixtures of W.sub.2 C and W.sub.3 C in admixture with tungsten.
In co-pending U.S. Application Ser. No. 07/153,738, now U.S. Pat. No. 4,855,188, such coatings are described in which an intermediate layer of substantially pure tungsten is used between the substrate and the mixture of tungsten and tungsten carbide outer layer to confer additional erosion and abrasion wear resistance characteristics on the composite coating system.
The coatings described in the co-pending applications are generally suitable for depositing on a wide range of substrates having both exterior and interior surfaces of large dimensions and of especially large cross-sectional area. They can be applied on inside surfaces of structures having small cross-sectional area, but they are deposited non-uniformly and generally fail to meet close tolerance requirements discussed earlier. The present invention is an improvement over the co-pending applications wherein wear resistant mixtures of tungsten and tungsten carbide coating systems are applied uniformly at low temperatures over the entire internal surfaces of structures of small dimensions such as a drill bushing or the like without distorting, deforming and degrading their mechanical properties by directing the mixture of reaction gases into contact with the heated internal surface. It has been found that by directing the reaction gas mixture into contact with the heated internal surface and controlling the reaction conditions, the internal surface can be uniformly coated to meet close tolerances. Furthermore, it has been found that any alteration in tolerances can easily be adjusted by lapping or other finishing techniques. It has also been found that such coating systems can confer substantial abrasion and erosion wear characteristics on the wear surfaces of such structures.